The invention relates to a method and apparatus for transporting workpieces during treatment.
In many processes where identical samples must be treated, e.g., by coating, spraying, etching, ion-implantation, etc., uniform treatment of each sample and across each sample is important. Generally, the source of treatment, e.g., the spray, evaporation source, or ion beam, is not perfectly uniform in intensity across its area of treatment, and to achieve uniform results either the source of the samples must be moved relative to each other during treatment. Further, in order to make sure that each sample is treated identically, it is conventional to treat a larger area than over which the samples move relative to the source and thus avoid "edge effects". This treatment of area not containing samples is very inefficient and can waste valuable treating material. A similar or greater efficiency is created if the source is held fixed and the samples move sequentially with respect to the source. The ideal system would be one which utilizes all of the spray or beam from the source and rearranges the sample array to achieve uniformity.
High efficiency is particularly important in processes using valuable products in expensive operations, such as the coating in vacuum of integrated circuit wafers with gold and other valuable material. In such treatments, typical wafers are two to three inches in diameter and 0.020 inches thick. They are coated in a vacuum chamber eight to thirty-six inches in diameter by means of a beam of gold molecules evaporated from a hot source.
Various devices have been developed for improving the uniformity of the twenty to one hundred wafers which are usually processed in each batch in treatments of this type. All such techniques are compromises in that the wafer does not duplicate the motion of every other wafer unless much of the beam is wasted. For example, devices in the prior art have been sold in which workpiece holders are moved in planetary or epicycloid rotation along a circular path. Devices are also sold in which holders mounted on a plate rotate while the plate itself is rotated about another axis. One difficulty is that some planets have a different exposure from other planets and thus uniformity is not assured. Gears have been utilized to drive the individual workpiece holders.
The present invention relates to a method and apparatus for resolving many of the problems which have existed in transporting workpieces as described above. According to the present invention, each of the workpieces is mounted on a holder and the holder moved along a guide slot forming a closed, non-circular path which covers substantially all of the surface altering treatment area, generally the area on which the beam is incident. The path thus necessarily is not a single straight line. The path can consist of straight line and curved segments which almost entirely fill any desired space, e.g., a rectangular area. Since the path is closed, each of the workpieces is subjected to the same identical treatment.
More particularly, in the embodiment described in detail below, a double sprocket chain is mounted along one side of the slot, while a fixed chain is mounted along the other side. The double sprocket chain is driven through a gear by a motor to cause movement of each of the individual workpieces which engage the respective first and second chains. Since the second chain is fixed or moves at a differential rate with regard to the double sprocket chain, the workpiece holders which mount the workpieces on the upper surfaces thereof also rotate during movement along the closed path. The workpiece holders are connected together by links which cover the slot portions not covered by the upper surfaces of the workpiece holders and the workpieces themselves. Cooling lines can be provided, e.g., below the base forming the bottom of the slot, and extending along the path of the slot to cool as necessary the workpiece holders and workpieces.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.